Put in anything you want, longer than eight chars.
The Passphrase is used to build and identify the generated keys
Keep the passphrase in mind, maybe you'll need it later again (e.g. for signing another key etc.)

I would like to know how SSl works, how the security certificates work with my server, what all the information I put in there (name, company, email, etc) is used for, and so on, in more details... Is there any documentation anywhere where that whole process is explained?